1. Field of Invention
The present invention relates to a polymer compound, a composition for an alignment film, an alignment film, an optical element, and an optical information writing/reading device.
2. Discussion of Background
Various optical recording media such as CD (compact disk), DVD (digital versatile disk) and a high density optical recording disk by blue light (hereinafter referred to as “BD”) have been developed, and optical information writing/reading devices to carry out recording of information on and/or reproduction of information from such optical recording media are widely used.
In such an optical information writing/reading device, an optical element such as a retardation plate or a diffractive element for shifting (polarizing, phase-adjusting, etc.) incident light (laser beam) from a light source, is usually used. Further, in an optical information writing/reading device, it is common to use, as a retardation plate, e.g. a quarter wavelength plate to shift linearly polarized light to circularly polarized light or a half wavelength plate to shift linearly polarized light to linearly polarized light having a polarization plane tilted by 90°.
Further, such an optical element for shifting incident light is also incorporated in a liquid crystal display device and is widely used as an absorption type circularly polarizing plate (λ/4 phase difference layer or λ/2 phase difference layer) or a linearly polarizing plate to constitute a liquid crystal display device, or a viewing angle compensation layer in various liquid crystal modes.
Here, an optical element such as a retardation plate or a diffractive element is usually prepared by using an optical anisotropic material having a liquid crystal material aligned. The optical anisotropic material is a material having an optical anisotropy such as a refractive index anisotropy derived from a mesogen structure. Further, it is important that an optical element prepared by using such an optical anisotropic material, has a uniform and proper Rd value (represented by Rd=Δn×d where Δn is a refractive index anisotropy and d is the thickness in the propagation direction of light).
As a method for producing such an optical anisotropic material, a method is known wherein a polymerizable liquid crystalline compound is polymerized in a state aligned by using an alignment film to obtain a film-form optical anisotropic material wherein such an aligned state is fixed. For example, Patent Documents 1 and 2 disclose that a (meth)acrylate compound having a liquid crystalline structure has a high reactivity, and therefore, when an optical anisotropic material is prepared therefrom by the above method, it is possible to obtain an optical anisotropic material having a uniform Rd value and is excellent also in transparency. Here, in the Patent Documents 1 and 2, a monofunctional (meth)acrylate compound is used.
However, the optical anisotropic film prepared by using such a (meth)acrylate compound having a liquid crystalline structure is not sufficient in the mechanical strength and heat resistance, and various proposals have been made for their improvements. For example, Patent Document 3 discloses a method of carrying out crosslinking by adding a polyfunctional (meth)acrylate having a liquid crystalline structure to the monofunctional (meth)acrylate compound. Further, Patent Documents 4 and 5 disclose a method for producing an optical element by a polymerizable liquid crystal composition composed solely of a polyfunctional acrylate having a liquid crystalline structure. However, there has been a problem that as the content of a polyfunctional (meth)acrylate increases, the adhesive strength with the adjacent alignment film tends to be poor, and peeling is likely to be caused by the inadequate adhesive strength.
Here, as an alignment film to align the liquid crystalline compound, a polyimide film is usually used, since it is excellent in the aligning performance to the liquid crystalline compound and at the same time has sufficient heat resistance and mechanical strength. Further, for the purpose of reducing the thickness of the alignment film, Patent Document 6 discloses an alignment film substrate employing a monomolecular film (LB film) of a fumaric acid diester polymer. Further, for the same purpose, Patent Document 7 discloses a method for subjecting a thin film of a fumaric acid diester polymer to aligning treatment by a rubbing method. However, like the polyimide alignment film, such an alignment film of a fumaric acid diester polymer was poor in the adhesion with a polyfunctional (meth)acrylate compound polymer, although it has the above-mentioned alignment performance, heat resistance and mechanical strength.
Patent Document 1: JP-A-7-17910
Patent Document 2: JP-A-8-3111
Patent Document 3: JP-A-11-80081
Patent Document 4: JP-A-2005-272560
Patent Document 5: JP-A-2008-9284
Patent Document 6: JP-A-02-214731
Patent Document 7: JP-A-03-21919